Elevators and hoists are equipped with safety devices, or governors, to prevent the hoist car that carries passengers or materials from falling in the event of mechanical or electrical failure. One such safety device is an overspeed, which is designed to progressively brake and eventually stop the car of a rack and pinion hoist and lock it in position if the car exceeds a predetermined speed (in either an upward or downward direction).
FIG. 1 is a cutaway view of an exemplary centrifugally-activated overspeed 101 already known in the art. The overspeed 101 has a brake pinion 102 which, when mounted in a hoist car, engages a rack affixed to the mast of an assembled hoist apparatus (not shown). This brake pinion 102 is constantly engaged with the rack and turns as the hoist car travels up and down the mast. The brake pinion 102 is attached to a brake shaft 103, which under normal operating conditions terminates at the apex of a brake cone 104 (sometimes referred to as the “inner drum”) and is encased in a bushing 105 to freely rotate. Attached to the brake shaft 103 and fitting inside the brake cone 104 adjacent to a backing plate 117 is a centrifugal mechanism 106 which is comprised of a main body or carrier 107, a bracing element 108, a centrifugal weight 109 (sometimes called a “dog”) that is free on one end 111 and is hinged 112 on the other end 113 which abuts the bracing element 108, and a tension spring 110 which holds the free end of centrifugal weight 109 close to the carrier when the overspeed 101 is not activated.
When the hoist car exceeds a predetermined speed, the overspeed 101 is activated to brake the hoist car. The overspeed 101 is activated when the revolutions per minute of the brake pinion 102 cause the brake shaft 103 and centrifugal mechanism 106 to rotate with enough centrifugal force that the tension spring 110 is unable to counteract the centrifugal force on the centrifugal weight 109 and its free end 111 displaces outward around its hinged end 113.
With reference to FIG. 2, the centrifugal weight 109 (shown in its non-displaced position on FIG. 2) is able, when displaced, to act as a pawl and catch the edge of a lip 114 which resides on the interior surface of brake cone 104. When the centrifugal weight 109 engages the lip 114, the brake cone 104 begins to spin and winds onto a threaded shaft 115 (see FIG. 1) and is pulled upward toward a brake drum 116 until the brake cone 104 frictionally engages a brake lining 118 on the brake drum 116 and the brake cone 104 seats against the brake drum 116. At this point, the brake shaft 103, and thus the brake pinion 102, can no longer rotate, and the overspeed 101 mounted inside the hoist car brings the car to a stop on the rack.
Overspeed devices 101 are routinely tested in the field upon hoist installation at a job site, but are also required to be periodically inspected, calibrated, and re-certified for continued safe use. Currently such inspection and certification procedures are undocumented and somewhat imprecise in that they depend upon human reaction to adjust and detect the trip speed of the centrifugal weight 109, for example. Further, no records of an individual overspeed's performance in calibration or certification tests are maintained or provided to the owner of the overspeed; all that is provided is a data tag with a certification expiration date for the current overspeed calibration.